This invention relates to electro-acoustic transducers. It is widely known that the most faithful acoustic reproduction of sounds, whether derived from recorded or broadcast signals, is obtained from transducers having acoustically stiff diaphragms which move as a piston, that is all parts of the diaphragm move together and there is no relative movement of one part of the diaphragm with respect to any other. This is very difficult to achieve in practice, however, and most diaphragms have some point in their operating range where they "break up" that is various parts start to vibrate at frequencies other than that or those at which the remainder of the diaphragm is vibrating, this is particularly the case when a transducer such as a loudspeaker is fed with a complex electrical signal containing a wide range of frequencies.
The present invention is particularly adapted for use in connection with loudspeakers of the type comprising a coil connected mechanically to the apex of a rigid conical diaphragm and moving in the magnetic field produced by a permanent magnet when fed with electrical signals representing the acoustic sounds which transducer is to reproduce.
In greater detail, optimum transformation from the mechanical energy of movement in the diaphragm to acoustic sound energy in the air occurs when the diaphragm moves as a whole, that is, when the relative movement of each portion of the cone with respect to any other portion thereof is zero. Because of the physical nature of the diaphragm and the wide range of frequencies of oscillation which the diaphragm has to reproduce, for example from 10 Hz to 40kHz, loudspeaker cones move as pistons only over a certain restricted part of the wide frequency range required for optimum transmission of electric to acoustic energy; the particular part of the frequency range being determined to some extent by the physical dimensions of the diaphragm.
At frequencies above a certain characteristic frequency the mechanical energy produced by the coil generates a wavefront in the structure of the cone or diaphragm which travels outwards longitudinally through the body of the diaphragm in a similar manner to ripples in water generated by a stone. The energy in the wavefront thus travels radially along the cone from the moving coil to the cone mouth which is supported by a cone surround. When the wavefront arrives at the cone mouth it encounters a discontinuity in the medium through which it is propagating. This discontinuity is either caused by the cone surround being of a different material or, if the cone surround is made from the same material as the cone, because corrugations have been introduced. If the cone mouth is held directly onto, say, the support frame of the loudspeaker, then this also presents a discontinuity in the propagating medium of the travelling wavefront. It is well-known that if a wavefront in a propagating medium encounters a discontinuity, then some of the energy in the wavefront will be dissipated at the discontinuity and some will be reflected back again, the precise direction of the reflected energy being dependent on the geometry of the propagating medium and the discontinuity. When multiples and sub-multiples of the wavelength of the frequency of energy in the wavefront coincide with the distance from the source of the energy i.e. the moving coil, to the discontinuity of the cone mouth i.e. the cone surround or cone mouth support, then the energy is reflected backwards and a standing wave is formed. This gives rise to an undesirable resonance or storage of energy and a marked change in the transformation of mechanical to acoustic energy occurs. This gives rise to colourations, or distortions in the transfer of energy from the coil to sounds heard by the ear.
The above mentioned break-up point comes when standing waves are set up in the cone. If the amplitude of oscillation of various parts of the cone surface is observed, for example by taking holograms with monochromatic light, the break-up patterns are seen to form variously shaped surfaces such as circumferential concentric rings, with perhaps a number of radial triangular shaped areas, adjacent areas, or concentric rings moving in opposite directions.
The severity of the standing wave patterns and the frequencies at which they occur depend on a number of factors, for example, the overall physical size of the diaphragm, that is the diameter at the largest point, the propagation characteristics of the material of which the cone is made, and the manner in which the cone mouth is supported. Of these the propagation characteristics of the cone material and the manner in which the mouth of the cone is supported are both extremely important.
Theoretically, of course, the propagation characteristics of the cone material can be selected such that, together with the characteristics of the mounting of the mouth of the cone, all the energy travelling radially from the moving coil through the body of the diaphragm is dissipated at the junction of the cone and the mount. If this condition is achieved then the cone is said to be critically terminated. More precisely the cone and surround may be viewed as an acoustic version of the electrical transmission line analogy. If the surround presents an acoustic impedance to the travelling wavefront, equal to the characteristic acoustic impedance of the cone, then no reflection will occur at the cone edge and no standing wave patterns will be set up.
In practice, however, the choice of suitable materials for the cone and cone mounting is limited by practical considerations, so that this ideal arrangement is very difficult to achieve.
The present invention relates to alternative methods of limiting or avoiding standing waves by modification of the propagation characteristics of the material from which the cone is made. It is known that the frequency response characteristics of a diaphragm can be improved by forming a plurality of holes or perforations in a diaphragm. Previous attempts to achieve such improvement, however, have been confined to the rather empirical formation of holes arranged in circles centered on the center of the diaphragm, apart from one attempt in which a single curved row of rather large holes was formed to a logarithmic curve or an involute curve. An attempt to achieve the same result was also made by forming a logarithmically curved ridge on the diaphragm. These attempts were not entirely successful, however, since insufficient attention was paid to the precise configuration, location and size of the perforations which are essential if the required effect is to be achieved.
It is also known to introduce plugs of a damping material into the holes in a diaphragm to provide further control over the formation of standing waves, but again completely satisfactory results have not been obtained.